Taotao Qiu

Bouncing universe with Quintom matter

The bouncing universe provides a possible solution to the Big Bang singularity problem. In this paper we study the bouncing solution in the universe dominated by the Quintom matter with an equation of state (EoS) crossing the cosmological constant boundary. We will show explicitly the analytical and numerical bouncing solutions in three types of models for the Quintom matter with an phenomenological EoS, the two scalar fields and a scalar field with a modified Born-Infeld action.

Nonsingular cosmology with a scale-invariant spectrum of cosmological perturbations from Lee-Wick theory

We study the cosmology of a Lee-Wick type scalar field theory. First, we consider homogeneous and isotropic background solutions and find that they are nonsingular, leading to cosmological bounces. Next, we analyze the spectrum of cosmological perturbations which result from this model. Unless either the potential of the Lee-Wick theory or the initial conditions are finely tuned, it is impossible to obtain background solutions which have a sufficiently long period of inflation after the bounce. More interestingly, however, we find that in the generic noninflationary bouncing cosmology, perturbations created from quantum vacuum fluctuations in the contracting phase have the correct form to lead to a scale-invariant spectrum of metric inhomogeneities in the expanding phase. Since the background is nonsingular, the evolution of the fluctuations is defined unambiguously through the bounce. We also analyze the evolution of fluctuations which emerge from thermal initial conditions in the contracting phase. The spectrum of gravitational waves stemming from quantum vacuum fluctuations in the contracting phase is also scale-invariant, and the tensor to scalar ratio is not suppressed.

Bouncing Galileon Cosmologies

We present nonsingular, homogeneous and isotropic bouncing solutions of the conformal Galileon model. We show that such solutions necessarily begin with a radiation-dominated contracting phase. This is followed by a quintom scenario in which the background equation of state crosses the cosmological constant boundary allowing for a nonsingular bounce which in turn is followed by Galilean Genesis. We analyze the spectrum of cosmological perturbations in this background. Our results show that the fluctuations evolve smoothly and without any pathology, but the adiabatic modes form a blue tilted spectrum. In order to achieve a scale-invariant primordial power spectrum as required by current observations, we introduce a light scalar field coupling to the Galileon kinetically. We find two couplings which yield a scale-invariant spectrum, one of which requires a fine tuning of the initial conditions. This model also predicts a blue tilted spectrum of gravitational waves stemming from quantum vacuum fluctuations in the contracting phase.

On perturbations of a quintom bounce

A Quintom universe with an equation-of-state crossing the cosmological constant boundary can provide a bouncing solution dubbed the Quintom Bounce and thus resolve the Big Bang singularity. In this paper, we investigate the cosmological perturbations of the Quintom Bounce both analytically and numerically. We find that the fluctuations in the dominant mode in the post-bounce expanding phase couple to the growing mode of the perturbations in the pre-bounce contracting phase.

CONSTRAINTS ON THE SOUND SPEED OF DYNAMICAL DARK ENERGY

In this paper we study in detail the sound speed —

A string-inspired quintom model of dark energy

c_s^2

Model of inflationary cosmology without singularity

, which is directly related to the classical perturbations — of dynamical dark energy (DE). We consider especially the case with an equation of state crossing the cosmological constant boundary, and show its implications for cosmic microwave background (CMB) anisotropy. With the present observational data on the CMB, the Type Ia supernova (SNIa) and galaxy clustering, we perform a global analysis to constrain the sound speed of DE, using the Markov chain Monte Carlo method. We find that the sound speed of DE is weakly constrained by current observations, and thus futuristic precision measurements of the CMB on a very large angular scale (low multipoles) are necessary.

Oscillating universe with quintom matter

Abstract We propose in this Letter a quintom model of dark energy with a single scalar field ϕ given by the Lagrangian L = − V ( ϕ ) ( 1 − α ′ ∇ μ ϕ ∇ μ ϕ + β ′ ϕ □ ϕ ) 1 / 2 . In the limit of β ′ → 0 our model reduces to the effective low energy Lagrangian of tachyon considered in the literature. We study the cosmological evolution of this model, and show explicitly the behaviors of the equation of state crossing the cosmological constant boundary.

Avoiding the Big-Rip Jeopardy in a Quintom Dark Energy Model with Higher Derivatives

In this paper, we consider a model of inflationary cosmology with inflation preceded by a bounce and study its primordial curvature perturbations. Our model gives rise to a primordial power spectrum with a feature of oscillation on large scales compared with the nearly scale-invariant spectrum generated by the traditional slow-rolling inflation model. This effect changes the cosmic microwave background temperature power spectrum and the large scale structure matter power spectrum. Further, with a detailed simulation we will show that in some detailed models this signal might be detectable in forthcoming observations, such as PLANCK and LAMOST.

G-bounce inflation: towards nonsingular inflation cosmology with galileon field

In this Letter, we study the possibility of building a model of the oscillating universe with quintom matter in the framework of 4-dimensional Friedmann-Robertson-Walker background. Taking the two-scalar-field quintom model as an example, we find in the model parameter space there are five different types of solutions which correspond to: (I) a cyclic universe with the minimal and maximal values of the scale factor remaining the same in every cycle, (II) an oscillating universe with its minimal and maximal values of the scale factor increasing cycle by cycle, (III) an oscillating universe with its scale factor always increasing, (IV) an oscillating universe with its minimal and maximal values of the scale factor decreasing cycle by cycle, and (V) an oscillating universe with its scale factor always decreasing